[SSS06-P03] Estimating centroid location and source dimension of the Te Araroa earthquake (Mw 7.1), New Zealand by analyzing direct and reflected tsunamis
Keywords:The 2016 Te Araroa earthquake, New Zealand, Tsunami, Coastal reflected wave, Centroid moment tensor
We estimated the centroid location using grid search approach. Fixing the magnitude, fault geometry and centroid depth (GCMT), we assumed the rectangular planar fault using the scaling law (Wells and Coppersmith, 1994). Then we searched for the centroid location that best reproduces the observed waveforms, based on the VR (variance reduction) between the observed and calculated waveforms. We first used the direct waves and obtained the centroid location at ~15 km north of the GeoNet centroid (~ 130 km northeast from the coast). However, the high-VR area (> 90% of the best solution’s VR) extended ~100 km in the WSW-ENE direction, suggesting the centroid location was not constrained well. We then used tsunami reflected from the coast to calculate VR, and obtained the centroid location at ~10 km northwest of the GCMT centroid (~ 80 km northeast from the coast). The extent of the high-VR area was reduced to ~40 km in the WSW–ENE direction and does not include the GeoNet and USGS centroids. This suggests that the GCMT is the most suitable solution for the centroid location, and the reflected waves contributed for constraining the centroid location.
We then searched for the earthquake source dimension by fixing the seismic moment and fault geometry to the GCMT value and assuming the ratio of source length L to width W such that L/W = 2 (rigidity μ = 40 GPa). With the direct waves alone, the best source length was obtained as L = 40 ± 20 km. On the other hand, when using both the direct and reflected waves, the source length was L = 50 ± 15 km. Although the source dimension is not constrained well, the upper limit of the possible source length was almost the same in both analyses, and the modeling result using the coastal reflection suggests that a smaller source dimension (L < ~ 30 km) is not plausible. Using the range of source dimensions obtained from the analysis of the coastal reflection, we calculated the stress drop Δσ of ~ 0.5 – 3.0 MPa, which is a typical value for earthquake stress drop (e.g., Kanamori and Anderson, 1978).